The present invention generally relates to methods and apparatuses for expansion forming and, more specifically, to a method and apparatus for expansion forming a workpiece using an external deformable supporting fixture.
Oxide-dispersion-strengthened (ODS) high temperature alloys offer combinations of high-temperature strength, oxidation resistance, and hot corrosion resistance that can not be obtained from other alloys. For example, INCONEL alloy MA 754 has been considered as one of the ODS alloys with greater potential in the next generation advanced gas-turbine hot section components such as turbine vanes and combustor liners. In such applications, the ODS alloys are typically used to fabricate LAMILLOY sheets. As is known in the art, LAMILLOY is a multilayered porous material designed for cooled airframe and propulsion system components. It features a labyrinth of holes and passages in a laminated assembly. LAMILLOY is produced by photochemical machining an array of pedestals and holes in two or more layers of sheet material and subsequently diffusion bonding the layers into the laminated sheet configuration.
One particular use of MA 754 LAMILLOY material is in the construction of a combustor outer liner 10 for a gas turbine engine, as illustrated in FIG. 1. The combustor liner 10 is formed by roll forming two LAMILLOY sheets into half cylinders (or conic sections) 12 and 14, brazing the half cylinders 12 and 14 together at two lap joints 16 and 18, and then expansion forming the brazed detail 10 into the required design dimension, as indicated by the dashed line 20 in FIG. 2.
The MA754 LAMILLOY material 12 and 14 is preferably joined by brazing lap joints 16 and 18 rather than welding the two half cylinders 12 and 14 together at butt joints, because welding processes result in melting of the base material. Welding is unacceptable in MA754 LAMILLOY material due to agglomeration of the oxide dispersoids in the melted region resulting in drastic reductions in high temperature strength and environmental resistance.
The joining of two MA754 LAMILLOY sheets is accomplished by brazing, which is performed at a temperature below the melting point of the base material. The brazing process requires a lap joint (overlapping ends of the material to be brazed) instead of the butt joint normally used in welding. Therefore, the two half-cylinders 12, 14 are configured with the two lap joints 16, 18 which are brazed to form a permanent joint.
As shown in FIG. 2, the combustor liner 10 initially has a flat-sided configuration; however, the design requirements call for the sides to exhibit the curved configuration indicated by the dashed lines 20. In order to achieve the configuration 20, the workpiece 10 is placed onto a radial expander 22 (see FIG. 3), which will exert radial forces upon the workpiece 10 until it assumes the configuration 20. As is known in the art, the expander 22 includes a plurality of jaws 24 which may be moved in a radial direction under, hydraulic pressure.
Referring to FIG. 4A, a top plan view of the lap joint 16, 18 is shown. When the expander 22 is operated, a biaxial tension state of stresses is produced in the directions shown by the arrows in FIG. 4C. Because the half cylinders 12, 14 lie in different planes in the area of the lap joint 16, 18, the braze joint plane 26 rotates through an angle 28 (see FIG. 4B), which can be as much as 30 degrees relative to the stress axis.
This joint rotation results in loading the braze joints 16, 18 in combination of shear, bending and direct tensile stresses. Stress analysis confirms that high stress concentrations exist in the LAMILLOY near the lap joints 16, 18 during expansion forming. As an illustrative example, a combustor liner 10 was formed from two halves 12, 14 of rolled MA 754 LAMILLOY. Two brazed lap joints 16, 18 in the liner 10, featured overlap dimensions of 0.250 inch wide and 6.750 inches in length. During expansion forming of this combustor liner 10, the LAMILLOY material failed in a ductile manner due to strain localization near one of the brazed lap joints 16, 18 with less than 2% bulk diametral expansion. The design deformation objective for this application was 4.5% diametral expansion.
A need exists for a method and apparatus that will allow expansion forming of a workpiece by minimizing the concentration of stresses in the workpiece, particularly stresses related to joint rotation out of the plane of the stress axis. The present invention is directed toward meeting this need.
The present invention generally relates to an external cylindrical or conic section shaped fixture which is tightly fit around the same shaped workpiece prior to expansion forming to a final dimension. During expansion forming, the external fixture is also deformed and is therefore designed for one-time use. The external cylindrical or conic section shaped fixture of the present invention is particularly useful for expansion forming of workpieces which contain brazed lap joints. The external fixture restrains out-of-plane joint rotation and, reduces the stress concentrations existing near the lap-joints. Additionally, the external fixture provides additional load carrying capability to allow better load distribution during expansion forming. No change is required to the expansion forming machinery or the inner expander dies (jaws) with use of the present invention. Because the external fixture fits tightly over the workpiece, applying a compressive force when the inner expander jaws expand during the forming process, the bending moment on the brazed lap joints is significantly reduced.
In one form of the invention, a method for expansion forming a workpiece is disclosed, comprising the steps of a) providing an expansion forming device; b) providing an external fixture; c) fitting the external fixture around the workpiece; d) fitting the workpiece and external fixture assembly around the expansion forming device, such that the workpiece is positioned between the expansion forming device and the external fixture; e) expanding the expansion forming device such that the workpiece and the external fixture are deformed; and f) removing the external fixture from the workpiece after step (e).
in another form of the invention, a method for expansion forming a workpiece is disclosed, comprising the steps of a) providing an expansion forming device; b) providing a fixture; c) coupling the workpiece to the expansion forming device; d) coupling the fixture to the workpiece; e) expanding the expansion forming device such that both the workpiece and the fixture are deformed; f) removing the fixture from the workpiece after step (e).